Snap ringed bit retainer ring

ABSTRACT

A bit retainer system includes a bit having a head and a shank, a driver sub disposed around the shank of the bit and adapted to engage a hammer case, a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer includes a split ring with a circumferential groove, and a secondary retainer disposed around the bit retainer, wherein the secondary retainer includes a snap ring configured to engage the circumferential groove.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation-in-Part of U.S. patent application Ser. No. 11/257,294, entitled “Secondary Retainer for a Downhole Hammer Bit” filed Oct. 24, 2005, which is incorporated by reference herein in its entirety.

BACKGROUND OF DISCLOSURE

1. Field of the Disclosure

The disclosure relates generally to percussion, or hammer, bits. More specifically, the disclosure relates to a secondary retainer for a downhole hammer bit.

2. Background Art

Percussion bit systems are often used in drilling or boring through the earth's surface. In a percussion bit system, a percussion hammer is used to drive the percussion bit into the ground using the reciprocating action of a piston to energize the bit.

FIG. 1 illustrates a conventional percussion bit assembly design 100. Percussion hammer assembly 100 comprises a hammer case 101 that connects to a lower end of a drill string (not shown) through a threaded pin connection 145. The lower end of the hammer case 101 is threadedly engaged with driver sub 102. A plurality of splines (not shown) disposed on the driver sub 102, engage a plurality of splines 115 disposed on a shank 112 of a hammer bit 110, and rotatatively drive the bit 110. A retainer, conventionally a split ring, 113 is disposed around an upper end of the shank 112 of the hammer bit 110 and abuts the driver sub 102. The split ring 113 retains the hammer bit 110 in the hammer assembly 100. The split ring 113 may be held in place, initially, by an elastic ring, or o-ring, 116 to facilitate assembly of the bit and driver sub with the hammer case. The split ring is confined by the inner wall of the hammer case 101 to maintain ring-to-bit engagement. The upper end of the hammer bit 110 includes a piston strike surface 148 and a foot valve, or blow tube, 147. The lower end of the hammer bit includes a head 111.

The hammer assembly includes a control tube 143 and an annular piston chamber 146. Pressurized air moves a piston 142 in a reciprocating motion inside the annular piston chamber 146. A check valve 144 is used to communicate one-way flow between the control tube 143 and the drill string (not shown). The lower end of the piston 142 is adapted to strike the piston strike surface 148, thereby imparting kinetic energy to the bit 110.

During certain operations performed with a hammer tool, it may be necessary to reverse drill pipe rotation, thereby causing the driver sub 102 to back off, or unthread, from the hammer case 101. Occasionally, a driver sub will unintentionally back off downhole due to torsional oscillations, known as “stick-slip”, of the drill string. If the driver sub backs off, the bit 110 and the driver sub 102 remain at the bottom of the borehole. When this occurs, the elastic ring 116 that restricts the movement of the split ring 113 during installation often fails and permits the split ring to fall away out of engagement with the bit. Consequently, the split ring does not fulfill the function of retaining the bit in the driver sub assembly. Therefore, when the driller attempts to screw back into the driver sub with the hammer case, the bit is left in the hole when the hammer is extracted. Sometimes the split ring segments lodge in between the driver sub 102 and the borehole wall, preventing the hammer case 101 from screwing back over the driver sub 102. The bit left in the hole has to be retrieved later through a costly fishing operation.

A segmented retainer sleeve 24, as shown in FIGS. 2 and 3 can be used to retain the bit 210 to the driver sub assembly after a driver sub backs off. FIGS. 2 and 3 show a conventional hammer assembly 200 comprising a driver sub 202, a segmented retainer sleeve 24, a hammer case 201, and a bit 210. U.S. Pat. No. 5,803,192 shows an example of such a conventional bit retainer.

The segmented retainer sleeve 24, as shown in FIGS. 2 and 3, includes a ring portion 24A, which seats about a reduced diameter segment 25 of the bit 210. Ring portion 24A, in combination with shoulder 24B, limits reciprocal travel of the drill bit 210. Skirt extension 26 has a lesser wall thickness than the ring portion 24A and slidably engages the bit 210. Skirt extension 26 nests within a counterbore 27 shown formed in the upper end of the driver sub 202. During a drilling operation, should the driver sub 202 back off, the segmented retainer sleeve 24 should remain in place about the bit because the skirt extension 26 is radially confined. Joining the hammer case 201 with the driver sub 202 after the driver sub 202 backs off may be accomplished by rotating the hammer case 201 on the driver sub 202. A drawback of this design is that, as a result of the counterbore 27 in the driver sub 202, the spline contact area between the splines 33 on the driver sub and the splines (not shown) on the bit 210 is reduced. A reduction in the spline contact area may result in rapid wear and breakage of the splines.

Accordingly, there exists a need for a downhole hammer bit retainer that retains the bit within the driver sub in the event the driver sub backs off downhole and that maintains the mechanical integrity of the driver sub and drill bit.

SUMMARY OF DISCLOSURE

In one aspect, embodiments disclosed herein relate to a bit retainer system including a bit having a head and a shank, a driver sub disposed around the shank of the bit and adapted to engage a hammer case, a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer includes a split ring with a circumferential groove, and a secondary retainer disposed around the bit retainer, wherein the secondary retainer includes a snap ring configured to engage the circumferential groove.

In other aspects, embodiments disclosed herein relate to a bit retainer system including a bit having a head and a shank, a driver sub disposed around the shank of the bit and adapted to engage a hammer case, a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer includes a split ring with a circumferential groove, and a secondary retainer disposed around the bit retainer, wherein the secondary retainer includes a plurality of preformed tabs disposed at selected portions along the retainer.

In other aspects, embodiments disclosed herein relate to a bit retainer system including a bit having a head and a shank, a driver sub disposed around the shank of the bit and adapted to engage a hammer case, a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer includes a split ring with a circumferential groove, and a secondary retainer disposed around the bit retainer, wherein the secondary retainer includes a plurality of preformed webs disposed at selected locations along the retainer.

In other aspects, embodiments disclosed herein relate to a bit retainer system including a bit having a head and a shank, a driver sub disposed around the shank of the bit and adapted to engage a hammer case, a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer includes a split ring with a circumferential groove, and a secondary retainer disposed around the bit retainer in the circumferential groove, wherein the secondary retainer includes a band having a plurality of elastically expandable buckles disposed on a first end and at least one protrusion disposed on a second end of the band.

Other aspects and advantages of the disclosure will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional view of a conventional hammer bit system design.

FIG. 2 shows a cross-sectional view of a conventional drill bit retainer design.

FIG. 3 shows an assembly view of a conventional drill bit retainer design.

FIG. 4 shows a bit retainer system in accordance with an embodiment of the present disclosure.

FIG. 5 shows a bit retainer system in accordance with another embodiment of the present disclosure.

FIGS. 6A and 6B show a bit retainer system in accordance with another embodiment of the present disclosure.

FIG. 7 shows a side view and a partial cross-section view of a bit retainer system in accordance with an embodiment of the present disclosure.

FIGS. 8A-8G show a bit retainer system in accordance with another embodiment of the present disclosure.

FIG. 9 shows a bit retainer system in accordance with an embodiment of the present disclosure.

FIGS. 10A and 10B show a bit retainer system in accordance with the embodiment of FIG. 9.

FIG. 11 shows a cross-sectional view of a bit retainer system in accordance with another embodiment of the present disclosure.

FIGS. 12A, 12B, and 12C show a bit retainer system in accordance with embodiments of the present disclosure.

FIG. 13 shows a bit retainer system in accordance with the embodiment of FIGS. 12A, 12B, and 12C.

FIGS. 14A, 14B, and 14C show a bit retainer system in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

When using a hammer drill, the driller may back off the driver sub, or occasionally, the driver sub may back off unintentionally. As used herein, the term “back off” means to unscrew drillstring components downhole. The drillstring components, which may include a drillpipe, a bottomhole assembly, a driver sub, etc., are coupled by various threadforms known as connections, or tool joints. Any of these drillstring components may unthread or back off. When a drillstring becomes stuck, the driver sub may unintentionally back off downhole, or it may be necessary to intentionally back off the drillstring from the driver sub to recover as much of the drilling string as possible. The back off may be intentionally accomplished by applying reverse torque and detonating an explosive charge inside a selected threaded connection. Alternatively, the back off may be intentionally accomplished by applying tension to the drillstring and detonating an explosive charge, thereby allowing the threads to slide by each other without turning. When a driver sub backs off, a bit retainer, usually a split ring configured to retain a drill bit in the driver sub during assembly, often separates from a bit shank. Accordingly, the split ring no longer performs the function of retaining the bit in the driver sub. Thus, when the driller attempts to screw back into the driver sub with the hammer case, the bit is left in the hole. In the event the bit head is lost in the hole, it may be necessary to cement the original wellbore and sidetrack. Sidetracking, or drilling a secondary wellbore away from an original wellbore, is an expensive operation and may cost around 20,000 to 50,000 dollars. Additionally, this results in a hole which thereafter has an undesirable dogleg. In one aspect, embodiments of the present disclosure relate to a secondary retainer for retaining the bit retainer to the drill bit.

Select embodiments of the present disclosure, as described below, include a bit retainer system. In one embodiment, the bit retainer system comprises a secondary retainer disposed around a bit retainer. In some embodiments, the secondary retainer comprises a perforated metallic deformable ring. The bit retainer is disposed around an upper end of a bit shank and abuts a driver sub. In some embodiments, the bit retainer comprises a segmented retainer ring. In the event the driver sub backs off the hammer assembly, a secondary retainer retains the segmented retainer ring to the drill bit shank. When the driller attempts to thread the hammer back on to the driver sub, the drill bit is retained in the driver sub, and the entire assembly may be reconnected to the hammer. Embodiments of the bit retainer system of the present disclosure may prevent the loss of a drill bit at the bottom of the hole and may prevent costly fishing operations to recover lost drill bits.

FIG. 4 shows an assembled bit retainer system in accordance with an embodiment of the present disclosure. In this embodiment, the bit retainer system 400 comprises a drill bit 410, a driver sub 402, a bit retainer (not shown), and a secondary retainer 413. In this embodiment, the secondary retainer 413 comprises a perforated metallic deformable ring. In select embodiments, the perforations may be tabs or webs.

FIG. 5 shows an exploded view of the bit retainer system of FIG. 4, in accordance with an embodiment of the invention. A driver sub 502 is disposed around a shank 512 of the bit 510. A plurality of splines 523 on the inside diameter of the driver sub 502 engage a plurality of splines 515 disposed axially on the outside diameter of shank 512 to rotatively drive the bit 510. In this embodiment, a bit retainer 505 comprises a segmented ring 508 disposed around shank 512 of the bit 510 and abutting the driver sub 502. A secondary retainer 513 is disposed around the bit retainer 505.

In this embodiment, the secondary retainer 513 comprises a perforated deformable ring 514. The perforated deformable ring 514 may be made from a flat strip of sheet metal that is punched to form a plurality of tabs 517 and then rolled and seam-welded 518. The plurality of tabs 517 are formed at selected locations around the circumference of the perforated deformable ring 514. One of ordinary skill in the art will appreciate that the number of tabs or webs formed and the selected locations of the tabs along the circumference of the perforated deformable ring may vary. The size, number, and location of the webs or tabs may depend on, for example, the dimensions of the drill bit or the load induced on the bit retainer and secondary retainer. In one embodiment, the plurality of tabs 517 align with a corresponding circumferential groove 519 formed in the bit retainer 505. In one embodiment, the circumferential groove 519 may be a continuous channel formed in the outside diameter of the bit retainer 505. In another embodiment, the circumferential groove 519 may be a series of grooves or notches that correspond to and align with the plurality of tabs 517.

After assembling the drill bit 510, driver sub 502, bit retainer 505, and the secondary retainer 513, the plurality of tabs 517 are plastically deformed inward, thereby engaging the circumferential groove 519 formed in the bit retainer 505. One of ordinary skill in the art will appreciate that the plurality of tabs 517 may be deformed by any method known in the art, for example, by pressing a screwdriver against tabs 517 to deform the tabs 517 radially inward. The engagement of the plurality of tabs 517 and the circumferential groove 519 secure the perforated deformable ring 514 to the bit retainer 505 and secure the bit retainer 505 in place around the bit 510. In the event the driver sub 502 backs off the hammer, the secondary retainer 513 retains the segmented ring 508 of the bit retainer 505 to the shank 512 of the bit 510. Accordingly, the drill bit 510 is retained inside the driver sub 502. The driller may recover the driver sub 502 and the drill bit 510 by reconnecting the hammer to the driver sub 502 by threading the hammer case (not shown) back onto the driver sub 502.

In accordance with an embodiment of the invention, the secondary retainer 513 may be removed from the bit retainer 505 by radially deforming the plurality of deformed webs radially outward, in the event the drill bit 510 needs to be replaced, or upon disassembly of the driver sub 502 and drill bit 510 from the hammer. The secondary retainer 513 may be reused, but this method may be time consuming. In accordance with an embodiment of the invention, the secondary retainer 513 may be cut off of the bit retainer 505 by aligning the area to be cut with the split of the segmented ring 508. In another embodiment, the plurality of tabs may be cut and pried apart, thereby allowing the secondary retainer 513 to be removed from the bit retainer 505.

In certain embodiments, the secondary retainer 513 includes a perforated ring 514 having a plurality of tabs 517 arranged at selected locations around the circumference of the perforated ring 514. The plurality of tabs 517 are preformed prior to installation onto the bit retainer 505. During installation, the secondary retainer 513 is expanded and positioned around the bit retainer 505, allowing the plurality of deformed tabs 517 to engage the circumferential groove 519 formed in the bit retainer 505 and essentially “snap” into place. The engagement of the plurality of tabs 517 and the circumferential groove 519 secures the perforated ring 514 to the bit retainer 505 and secures the bit retainer 505 in place around the bit 510. In the event the driver sub 502 backs off the hammer, the secondary retainer 513 retains the segmented ring 508 of the bit retainer 505 to the shank 512 of the bit 510. Accordingly, the drill bit 510 is retained inside the driver sub 502. The driller may recover the driver sub 502 and the drill bit 510 by reconnecting the hammer to the driver sub 502 by threading the hammer case (not shown) back onto the driver sub 502.

In another embodiment, as shown in FIG. 6A, the bit retainer system 600 comprises a drill bit 610, a driver sub 602, a bit retainer 605, and secondary retainer 613. In this embodiment, the bit retainer 605 comprises a segmented ring 608 and the secondary retainer 613 comprises a small gage wire 621. The small gage wire 621 is wrapped around the segmented ring 608 and disposed in a circumferential groove 619 on the outside diameter of the segmented ring 608. The small gage wire 621 retains the bit retainer 605 in its location around the drill bit in the event the driver sub backs off.

FIG. 6B shows a cross section view of a portion of the bit retainer system 600 of FIG. 6A. In this embodiment, the secondary retainer 613 comprises a small gage wire 621 wrapped around a bit retainer 605 and disposed in a circumferential groove 619 formed on the outside diameter of the bit retainer 605. In one embodiment, the ends (not shown) of the small gage wire 621 may be twisted around each other to prevent the small gage wire 621 from separating from the bit retainer 605. One of ordinary skill in the art will appreciate that the size of the wire and the number of wraps of wire may vary depending on, for example, the dimensions of the drill bit or the dimensions of the bit retainer.

In another embodiment, a secondary retainer in accordance with the present invention may be formed as shown in FIG. 7, wherein a secondary retainer 713 is formed with a plurality of tabs 717 and is disposed around bit retainer 705. A circumferential groove 719 is formed in the outside diameter of bit retainer 705. Once the secondary retainer 713 is assembled around the bit retainer 705, the plurality of tabs 717 are plastically deformed inward 722 and engage the circumferential groove 719. One of ordinary skill in the art will appreciate that the plurality of tabs 717 may be deformed by any method known in the art, for example, by pressing a screwdriver against tabs 717 to deform the tabs 717 radially inward. In one embodiment, as shown in FIG. 7, the tabs 717 may be formed in a hemispherical or crescent-like shape with a corresponding hemispherical or crescent-like shaped opening. The shape of the tabs is not limited to that shown in FIG. 7. One of ordinary skill in the art will appreciate that any shape tab that may be plastically deformed inward to engage a groove on the bit retainer may be used, without departing from the scope of the invention. For example, a rectangular shaped tab or a circular shaped tab may be used.

In certain embodiments, a secondary retainer may be formed as shown in FIG. 7, wherein the secondary retainer 713 is formed with a plurality of tabs 717 that are preformed prior to installation around the bit retainer 705. During installation, the secondary retainer 713 may be expanded and positioned over the bit retainer 705, the plurality of deformable tabs 717 engaging the circumferential groove 719. Further, edges of the circumferential groove 719 may include chamfers 723 to allow the preformed tabs 717 to engage the groove 719.

In another embodiment, as shown in FIGS. 8A, 8B, and 8C, a secondary retainer comprises a band 830 with a buckle 832 formed into the band. As shown in FIG. 8A, the buckle 832 may be integrally formed into the band 830. One of ordinary skill in the art will appreciate, however, that the buckle may be attached by means such as welding, riveting, crimping, or other similar means. FIG. 8B shows a cross-section view taken through the assembled buckle and part of the bit retainer 805. The bit retainer 805 may be a segmented ring similar to the bit retainer 605 shown in FIG. 6A. In this embodiment, the band 830 is placed in the circumferential groove 819 of the bit retainer 805 and wrapped around the bit retainer 805. The end 831 of the band 830 is then threaded through the buckle 832 and folded back, shown at 835, over the buckle 832. The band is then secured by at least one deformable tab 833, as shown in FIG. 8C. The at least one deformable tab 833 is plastically deformed over the end 832 of the band 830. As shown in FIG. 8B, the band 830 and buckle 832 may be protected from damage during handing and in use through at least one lip 840 formed into the bit retainer 805. The band 830 retains the bit retainer 805 to the bit (not shown), thereby preventing separation of the bit from the driver sub (not shown) in the event the driver sub backs off.

In another embodiment, as shown in FIGS. 8D-8G, a secondary retainer comprises a band 830 with buckles 832 on a first end of the band 830 as shown in FIG. 8D, and protrusions or “bumps” 834 on a second or opposite end of the band 830, shown in FIG. 8E. One of ordinary skill in the art will appreciate that the buckle 832 and protrusions 834 may be attached to or formed on the band 830 by any means such as welding, riveting, crimping, or other similar means. During installation, the secondary retainer 813 is placed into the circumferential groove 819 (shown in FIG. 8G) around the bit retainer 805, and the end of the band 830 with the protrusions 834 is passed through the buckles 832 of the opposite end of the band 830. As the protrusions 834 pass through the buckles 832, either the buckles 832, the protrusions 834, or both may elastically deform. After the protrusions 834 are fully engaged with the buckles 832, the protrusions 834 are prevented from “backing out” or passing back through the buckles 832 as shown in FIG. 8F by shoulders 835 of the protrusions 834 that abut the buckles 832. This arrangement prevents the secondary retainer 813 from loosening from around the bit retainer 805. As previously described, the band 830 may be protected by a lip 840 formed into the bit retainer 805

In another embodiment, shown in FIG. 9, a secondary retainer 913 formed with a plurality of webs 937 is disposed around bit retainer 905. A circumferential groove 919 is formed in the outside diameter of bit retainer 905. Once the secondary retainer 913 is assembled around the bit retainer 905, the plurality of webs 937 are plastically deformed inward 922 and engage the circumferential groove 919. In the embodiment shown in FIG. 9, the plurality of webs 937 may be formed by punching holes into a flat strip of sheet metal. The punched strip of sheet metal may then be rolled and seam-welded. The shapes of the holes punched and the corresponding tabs are not limited to that shown in FIG. 9. One of ordinary skill in the art will appreciate that any shape punch or web may be used without departing from the scope of the invention so long as the webs may be plastically deformed inward to engage a groove on the bit retainer. One of ordinary skill in the art will appreciate that the plurality of webs 937 may be deformed by any method known in the art, for example, by pressing a screwdriver against the webs 937 to deform the webs 937 radially inward.

In certain embodiments, the secondary retainer 913 may be formed with a plurality of webs 937 that are preformed prior to installation. During installation, the secondary retainer 913 may be expanded and positioned around bit retainer 905, thereby allowing the preformed webs to engage the circumferential groove 919. The webs 937 may be preformed using any shape punch as previously mentioned, or any other means known to a those skilled in the art.

FIG. 10A shows a perspective view of a secondary retainer 1013, formed in accordance with the embodiment shown in FIG. 9, disposed around a bit retainer 1005. Bit retainer 1005 comprises a split ring 1008. Secondary retainer 1013 is formed from a flat strip of sheet metal that is punched to form a plurality of webs 1037, rolled, and seam-welded 1018. The plurality of webs 1037 are then deformed radially inward 1022, shown in FIG. 10B, thereby engaging a circumferential groove 1019 on the bit retainer 1005.

In another embodiment, shown in FIG. 11, a secondary retainer 1113 is disposed around the outside diameter of the bit retainer 1105. In this embodiment, the secondary retainer 1113 abuts a lip 1140 formed in the bit retainer 1105. The lip 1105 protects the retainer ring 1113 from being displaced or damaged during assembly of the driver sub and drill bit into the hammer case. The secondary retainer 1113 may contain a plurality of webs or tabs formed so as to be deformed inward to engage the circumferential groove 1119 formed on the outside diameter of the bit retainer 1105.

In another embodiment, shown in FIGS. 12A-12C, a secondary retainer 1213 may be formed from a flat strip of sheet metal formed with a plurality of tabs 1217 disposed on at least one circumferential edge 1252, 1253. The flat strip of metal may then be rolled and seam-welded. As shown in FIG. 12B, the plurality of tabs 1217 may be disposed on both circumferential edges 1252, 1253 of the secondary retainer 1213, or as shown in FIG. 12C, the plurality of tabs 1217 may be formed on one circumferential edge 1253. The secondary retainer 1213 is disposed around a bit retainer 1205. In this embodiment, the plurality of tabs 1217 are deformed inward to engage with a corresponding angled surface 1242 on the bit retainer 1205.

FIG. 13 shows a perspective view of a secondary retainer 1313, formed in accordance with the embodiment shown in FIGS. 12A-12C, disposed around a bit retainer 1305. Bit retainer 1305 comprises a split ring 1308. Secondary retainer 1313 is formed with a plurality of tabs 1317 formed on one circumferential edge 1353. The plurality of tabs 1317 are deformed radially inward and engage with a corresponding angled surface 1342 on the bit retainer 1305. The secondary retainer 1313 engages with the bit retainer 1305 such that, in the event the driver sub backs off, the secondary retainer 1313 retains the segmented rings of the bit retainer in place around the drill bit. Accordingly, when the driver sub is reconnected to the hammer, the drill bit remains assembled to the driver sub.

In select embodiments, as shown in FIGS. 14A and 14B, a secondary retainer 1413 may be a snap ring 1421 used to secure bit retainer, or split rings 1405. As used herein, snap ring 1421 may be configured as a circular wire with a gap 1417 between two ends which allows snap ring 1421 to expand and contract as necessary. As shown in FIG. 14B, split ring 1405 may be configured with a circumferential groove 1419 to accept snap ring 1421 (FIG. 14A). As such, snap ring 1421 may be assembled onto split ring 1405 by initially expanding snap ring 1421 to fit over the outer circumference of split ring 1405, positioning snap ring 1421 over groove 1419, and allowing snap ring 1421 to contract into groove 1419. FIG. 14C shows an assembled bit retainer system in accordance with embodiments of the present disclosure. Bit retainer system 1400 includes a drill bit 1410, driver sub 1402, and split ring 1405 having a circumferential groove 1419 configured to receive a snap ring (not shown).

Snap ring 1421 may include various cross-sectional configurations, including, but not limited to, circular, square, trapezoidal or other shapes known to those skilled in the art. One of ordinary skill in the art will understand the appropriate sizing of snap ring 1421 and circumferential groove 1419 that is required to hold split ring 1405 together. Further, snap ring 1421 may be manufactured from metal or other materials known to a person skilled in the art. Further, in certain embodiments, snap ring 1421 may have a coating, such as a corrosion resistant coating, applied to an outer surface. In still further embodiments, snap ring 1421 may be formed from a magnetic material, or alternately, have a magnetic device attached by any method known in the art. The magnetic features may help to hold snap ring 1421 on more securely about split rings 1405.

Embodiments of the disclosure may include one or more of the following advantages. A bit retainer system with a secondary retainer to retain the bit retainer in a position necessary for retaining the bit to the driver sub during assembly of the hammer. A bit retainer system with a secondary retainer to retain the bit retainer in a position necessary for retaining the bit to the driver sub in the event the driver sub backs off. Accordingly, the bit retainer system may decrease the time for recovering a driver sub and drill bit. Additionally, the bit retainer system may decrease the cost of reconnecting the driver sub and drill bit to the hammer after the driver sub backs off. Further, embodiments disclosed herein may provide a secondary retainer capable of withstanding increased downhole temperatures, thereby eliminating failures due to temperatures downhole.

A metal snap ring as described above, may advantageously provide more reliable means of holding the bit retaining rings together during assembly or disassembly and may also hold together the bit retainer in case of a downhole back off. Such an occurrence may leave “junk” or unwanted material in the hole resulting in possibly a costly “fishing” job, or a blind attempt to retrieve components or trash, as well as non-productive drilling time. As is well known in the industry, any downtime costs large amounts of money and is avoided at all costs.

While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the disclosure as disclosed herein. Accordingly, the scope of the disclosure should be limited only by the attached claims. 

1. A bit retainer system comprising: a bit having a head and a shank; a driver sub disposed around the shank of the bit and adapted to engage a hammer case; a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer comprises a split ring with a circumferential groove; and a secondary retainer disposed around the bit retainer, wherein the secondary retainer comprises a snap ring configured to engage the circumferential groove.
 2. The bit retainer system of claim 1, wherein the snap ring is metal.
 3. The bit retainer system of claim 1, wherein the snap ring is magnetic.
 4. The bit retainer system of claim 1, wherein the snap ring is elastically expandable.
 5. A bit retainer system comprising: a bit having a head and a shank; a driver sub disposed around the shank of the bit and adapted to engage a hammer case; a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer comprises a split ring with a circumferential groove; and a secondary retainer disposed around the bit retainer, wherein the secondary retainer comprises a plurality of preformed tabs disposed at selected portions along the retainer.
 6. The bit retainer system of claim 5, wherein the secondary retainer is configured to elastically expand, thereby allowing the preformed tabs to engage the circumferential groove of the split rings.
 7. The bit retainer system of claim 5, wherein the split rings comprise a chamfer on at least one edge of the circumferential groove.
 8. A bit retainer system comprising: a bit having a head and a shank; a driver sub disposed around the shank of the bit and adapted to engage a hammer case; a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer comprises a split ring with a circumferential groove; and a secondary retainer disposed around the bit retainer, wherein the secondary retainer comprises a plurality of preformed webs disposed at selected locations along the retainer.
 9. The bit retainer system of claim 8, wherein the secondary retainer is configured to elastically expand, thereby allowing the plurality of preformed webs to engage the circumferential groove of the split rings.
 10. The bit retainer system of claim 8, wherein the split ring comprises a chamfer on at least one edge of the circumferential groove.
 11. A bit retainer system comprising: a bit having a head and a shank; a driver sub disposed around the shank of the bit and adapted to engage a hammer case; a bit retainer disposed around the shank of the bit and abutting an upper end of the driver sub, wherein the bit retainer comprises a split ring with a circumferential groove; and a secondary retainer disposed around the bit retainer in the circumferential groove, wherein the secondary retainer comprises a band having a plurality of elastically expandable buckles disposed on a first end and at least one protrusion disposed on a second end of the band.
 12. The bit retainer system of claim 11, wherein the protrusions are configured to engage the buckles and secure the band around the bit retainer. 